core-deps/glibmm-2.4.bst et al.: Switch to Meson

The following packages can now be built with Meson:

• core-deps/glibmm-2.4.bst
• core-deps/libsigcplusplus-2.0.bst
• core-deps/pangomm-1.4.bst
• world/glibmm.bst
• world/libsigcplusplus.bst
• world/pangomm.bst